This application is based on Japanese Patent Application No. 11-353539 filed Dec. 13, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a braking system for an automotive vehicle, and more particularly to an improvement of a braking system of a type wherein a brake is operated to produce a braking force corresponding to an output of a brake operating device.
2. Discussion of Related Art
JP-A-8-230634 discloses an example of a vehicle braking system of the type indicated above, which is provided with a brake operating device in the form of a vacuum booster, which is one type of a booster. The vacuum booster is arranged to receive an input force from a brake operating member in the form of a brake pedal upon operation of the brake pedal, and boost the received input force on the basis of a negative pressure, so that the boosted input force is transmitted as an output of the vacuum booster to a master cylinder, in order for the master cylinder to pressurize a working fluid. The pressurized fluid is delivered from the master cylinder to a brake cylinder of a wheel brake, so that the wheel brake is activated. The braking system disclosed in the above-identified publication is further adapted such that a pressurized fluid delivered by a pump is supplied to the wheel brake cylinder so that the fluid pressure in the wheel brake cylinder is made higher than that in the master cylinder, whereby the wheel brake generates a braking force larger than that based on the fluid pressure in the master cylinder. The pressurized fluid is delivered from the pump to the wheel brake cylinder when the brake operating member is rapidly operated to apply an abrupt brake to the vehicle, as well as when the wheel brake suffers from a phenomenon so-called xe2x80x9cfadexe2x80x9d wherein the friction coefficient of a friction member such as a friction pad or brake lining is excessively lowered. A determination that the application of an abrupt brake is required is made when the fluid pressure of the master cylinder detected by a suitable pressure sensor is higher than a predetermined value and when a rate of increase of the fluid pressure of the master cylinder has exceeded a predetermined value. When the requirement for the abrupt brake application is detected, the fluid pressurized by the pump is supplied to the wheel brake cylinder so that the braking force generated by the wheel brake when the brake pedal is abruptly operated is made larger, than when the brake pedal is normally operated.
In the braking system disclosed in the above-identified publication wherein the brake operating device such as a vacuum booster is disposed between the brake operating member and the master cylinder, there is a risk of a delay in the determination that the application of an abrupt brake is required, due to a delay in the operation of the brake operating device. For instance, the vacuum booster includes: a housing; a power piston which is axially sidably received within the housing and which cooperates with the housing to define a low-pressure chamber formed on the front side of the power piston and connected to an air intake system of an engine of the vehicle, and a variable-pressure chamber formed on the rear side of the power piston; an input member operatively connected to the brake operating member; an output member operatively connected to a pressurizing piston of the master cylinder; and a switch valve operable between a retracted position for fluid communication of the variable-pressure chamber with the low-pressure chamber, on the basis of a relative movement of the input and output members, an advanced position for fluid communication of the variable-pressure chamber with the atmosphere, and an intermediate position for inhibiting the fluid communication of the variable-pressure chamber with both of the low-pressure chamber and the atmosphere. In this case, the input force is received by the input member, and the output force is applied from the output member to the pressurizing piston of the master cylinder. The operational delay of the vacuum booster indicated above is a delay of the application of the output force to the pressurizing piston of the master cylinder with respect to the application of the input force to the input member, which delay may take place due to a delayed flow of the atmosphere into the variable-pressure chamber or a relatively low degree of vacuum in the low-pressure chamber (a relatively small difference of the pressure in the low-pressure chamber from the atmospheric pressure). The operational delay tends to increase with an increase in the operating speed of the brake operating member. Accordingly, the detection of the requirement for an abrupt brake application tends to be delayed when the brake operating member is operated at a relatively high speed. That is, a relatively rapid operation of the brake operating member tends to cause a delay in the moment at which is satisfied the above-indicated condition that the fluid pressure of the master cylinder is higher than a predetermined value and that a rate increase of the fluid pressure of he master cylinder has exceeded a predetermined value. There is a similar tendency also where a brake operating device of any other type such as a hydraulic booster is disposed between the brake operating member and the master cylinder, although there is some difference in the degree of the operational delay in the two cases indicated above.
The braking system disclosed in the above-identified publication is not provided with any means for solving the operational delay of the vacuum booster. As discussed above, the brake operating device such as the vacuum booster as the typical example may suffer from the operational delay, which prevents a delay in the increase of the master cylinder pressure concurrently with an increase in the operating force of the brake operating member (hereinafter referred to simply as xe2x80x9cbrake operating forcexe2x80x9d, where appropriate). This delay in the increase of the master cylinder pressure with respect to the brake operating force necessarily results in an increase in the required brake operating force. However, the conventional braking system is not provided with any means for dealing with this drawback.
It is therefore an object of the present invention to provide a braking system for an automotive vehicle, which is less likely to suffer from a delay in the detection of a requirement for an abrupt brake application and an insufficient amount of the brake operating force, even in the presence of an operational delay of a brake operating device disposed between the brake operating member and the master cylinder.
The above object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) A braking system for braking a wheel of an automotive vehicle, including a brake for braking said wheel and an operating device operable to produce an output corresponding to an input thereof, for activating the brake according to the output, and wherein the output of the operating device is delayed with respect to the input depending upon an operating condition of the braking system, the braking system comprising:
a brake-operating-force increasing device operable on the basis of a delay of the output of the operating device with respect to the input, to increase an operating force of the brake to a value which is larger than a value corresponding to the output of the operating device.
In the braking system constructed according to the above mode (1) of this invention, the brake-operating-force increasing device is operated on the basis of a delay of the output of the operating device with respect to its input, to increase the operating force of the brake to a value larger than a value corresponding to the output of the operating device. The above-indicated xe2x80x9cvalue corresponding to the output of the operating devicexe2x80x9d includes a delay of the output with respect to the input (operational delay of the operating device). Since the brake-operating-force increasing device is operated on the basis of this delay, the operating force of the brake increased by the brake-operating-force increasing device can be increased to a value larger than the value corresponding to the delay output of the operating device.
The brake-operating-force increasing device may be adapted to increase the operating force of the brake to a value larger than a value corresponding to the input of the operating device, or may be adapted to eliminate or reduce an amount of insufficiency of the operating force of the brake (a shortage of an actual value of the brake operating force with respect to a nominal value as represented by the input of the operating device), as described below with respect to the following mode (2) of the invention.
Where the brake-operating-force increasing device is adapted to increase the operating force of the brake to a value higher than a value corresponding to the input of the operating device (as represented by an operating force of a brake operating member), a determination as to whether an abrupt brake application to the vehicle is required or not is effected on the basis of the operational delay of the operating device. The operational delay of the operating device such as a vacuum booster is initiated when an operation of the operating device (an operation of a brake operating member) is initiated, and increases with a speed at which the brake operating member is operated, as described above with respect to the prior art. Accordingly, the requirement for an abrupt brake application to the vehicle can be detected at a relatively early point of time, on the basis of the operational delay of the operating device. Upon detection of the requirement for an abrupt brake application to the vehicle, the brake-operating-force increasing device is activated to increase the operating force of the brake to a value larger than a value corresponding to the input of the operating device, the later value being obviously larger than a value corresponding to the output of the operating device. Accordingly, not only the operational delay of the brake is reduced, but also the brake is operated with a larger operating force than in a normal braking operation, so that the required braking distance of the vehicle can be reduced. The brake-operating-force increasing device according to this arrangement may be referred to as an abrupt braking control device.
Where the brake-operating-force increasing device is adapted to increase the operating force of the brake so as to prevent or reduce an amount of insufficiency of the operating force of the brake, the problem of insufficiency of the operating force of the brake due to the operational delay of the operating device is mitigated or eliminated. The brake-operating-force increasing device according to this arrangement may be referred to as an operational-delay reducing device. Where the operating device is a vacuum booster, an upper limit of the operational delay of the vacuum booster is determined by taking account of a flow of the atmosphere into the variable-pressure chamber, and other factors which may cause the operational delay, and the brake-operating-force increasing device (operational-delay reducing device) is activated when it is found that the amount of the operational delay is larger than the determined upper limit, so as to totally or partially eliminate the problem of insufficiency of the operating force of the brake.
Where the delay of the output of the operating device with respect to the input is continuously detected, the following points are recognized. Namely, the operational delay is initiated almost simultaneously with the initiation of an operation of the operating device, and therefore the operational delay is detected at a relatively early point of time where the upper limit is determined to be relatively small. In this case, the operational delay of the brake can be accordingly reduced. On the other hand, however, the relatively small upper limit results in a relatively high frequency of operation of the operational-delay reducing device (brake-operating-force increasing device). Where the upper limit is determined to be relatively large, on the other hand, the frequency of operation of the operational-delay reducing device is lowered, but the operational delay of the brake is increased. Therefore, it is desirable that the upper limit of the operational delay of the operating device be determined so as not to cause an excessively high frequency of operation of the operational-delay reducing device and an excessively large amount of operational delay of the brake.
The operational delay of the operating device may be detected at a predetermined opportunity or opportunities. For instance, the operational delay is detected by detecting the output of the operating device when the input has increased to a predetermined value or values, or by detecting the input when the output has increased to a predetermined value or values. This arrangement, which does not require continuously detection of the operational delay of the operating device, is effective to reduce the operating load of a computer used for the braking system.
The brake may be a hydraulically operated brake including a hydraulic cylinder operable with a pressurized fluid, or an electrically operated brake including an electrically operated actuator.
(2) A braking system according to the above mode (1), wherein the brake-operating-force increasing device includes an abrupt braking control device operable to increase the operating force of the brake to a value larger than a value corresponding to the input of the operating device.
In the braking system according to the above mode (2) wherein the operating force of the brake is increased to a value larger than a value corresponding to the input of the operating device, so that the increased operating force changes with the amount of operational delay of the operating device. Accordingly, not only the insufficiency of the operating force of the brake due to the operational delay of the operating device is at least reduced, but also the brake is operated with the operating force required to apply an abrupt brake to the vehicle.
(3) A braking system according to the above mode (2), further comprising a brake operating member manually operable with an operating force, and wherein the operating device includes a booster which receives the operating force of the brake operating member as the input and boosts the input into the output, and the abrupt braking control device is operable to increase the operating force of the brake to a value larger than a value corresponding to the operating force of the brake operating member.
(4) A braking system according to any one of the above modes (1)-(3), further comprising a brake operating member manually operable with an operating force, and wherein the operating device includes a booster which receives the operating force of the brake operating member as the input and boosts the input into the output, and the brake-operating-force increasing device includes a braking-delay reducing device operable to increase the operating force of the brake such that the operating force of the brake approaches to a value corresponding to the operating force of the brake operating member.
In the braking system according to the above mode (4) which includes the feature of the above mode (2), the brake-operating-force increasing device includes both the abrupt braking control device and the braking-delay reducing device.
(5) A braking system according to any one of the above modes (1)-(4), wherein the brake-operating-force increasing device includes an output-delay detecting portion operable to detect the delay of the operating device on the basis of actual values of the input and the output of the operating device.
The operational delay of the operating device can be detected by detecting the actual values of the input and output of the operating device.
(6) A braking system according to the above mode (5), wherein the output-delay detecting portion includes a portion operable to detect the delay of the operating device on the basis of a relationship between the input and the output of the operating device at a predetermined point of time after a moment of initiation of an operation of the operating device.
The predetermined point of time may be a point of time a predetermined time after the moment of initiation of an operation of a brake operating member by an operator of the vehicle, or a point of time at which the input or output of the operating device has increased to a predetermined value.
The operational delay of the operating device may be continuously detected after the moment of initiation of an operation of the brake operating member by the vehicle operator. To assure efficient detection of the operational delay, the detection is preferably effected at a predetermined point of time, for instance, at a point of time at which a relatively large amount of the operational delay is expected. When or immediately after the operation of the operating device, its output is unstable, and the accuracy of detection of the operational delay is therefore likely to be deteriorated. The accuracy of detection may be increased if the detection is effected at a certain point of time at which the output is expected to be stabilized.
(7) A braking system according to the above mode (5) or (6), wherein the output-delay detecting portion includes an activating device which is operable to initiate an operation of the brake-operating-force increasing device when the activating device has detected that a difference between an ideal value of the output of the operating device which corresponds to an actual value of the input according to a static ideal relationship between the input and the output and an actual value of the output corresponding to the actual value of the input is not smaller than a predetermined threshold value.
When the operating device has an operational delay, the actual value of its output is smaller than the ideal value. Therefore, the operational delay is represented by a difference between the actual and ideal values of the output of the operating device. When the difference is larger than a predetermined threshold value or upper limit, this means that amount of operational delay is larger than a predetermined upper limit. In view of this fact, the braking system according to the above mode (7) is adapted to initiate an operation of the brake-operating-force increasing device when the difference of the actual value of the operating device with respect to the ideal value is found to be equal to or larger than the predetermined threshold value. Thus, the brake-operating-force increasing device is activated only when its operation is required to increase the operating force of the brake.
Where the operating device is a booster such as a vacuum booster, the actual value of the its output per se may be detected. However, the fluid pressure of a master cylinder operatively connected to the booster may be detected in place of the output of the operating device. Usually, it is easier to detect the master cylinder pressure than the output of the booster. Accordingly, the output-delay detecting portion is available at a lower cost where the operational delay of the operating device is detected by detecting the master cylinder pressure, than by detecting the actual output of the booster. Where the input of the booster is received from a manually operable brake operating member such as a brake pedal, the actual value of the input of the booster may be detected for comparison with the actual value of its output to detect the amount of operational delay of the booster. However, the operating force acting on the brake operating member may be detected in place of the actual input of the booster per se.
(8) A braking system according to the above mode (7), wherein the operating device is a vacuum booster operable to boost an input thereof into an output thereof by utilizing a negative pressure, and the output-delay detecting portion includes a threshold determining portion for determining the threshold value of the difference such that the threshold value is larger when a difference of a negative pressure from an atmospheric pressure is relatively small than when the difference of the negative pressure from the atmospheric pressure is relatively large.
The low-pressure chamber of the vacuum booster is connected to a negative pressure source in the form of the intake pipe or manifold of an engine of the vehicle, and the negative pressure in the low-pressure chamber varies depending upon the operating condition of the engine. When the negative pressure is relatively close to the atmospheric pressure, the operational delay of the booster with respect to an operation of the brake operating member is larger than when the negative pressure is relatively close to the absolute vacuum. If the threshold value of the difference between the actual and ideal values of the output of the booster is constant irrespective of the negative pressure in the booster, the operational delay of the booster is relatively easily detected to be excessive while the difference of the negative pressure from the atmospheric pressure is relatively small. According to the above mode (8), however, the determination as to whether the operational delay of the booster is excessive or not can be adequately effected irrespective of a change in the negative pressure in the low-pressure chamber of the booster.
(9) A braking system according to any one of the above modes (1)-(8), wherein the brake-operating-force increasing device includes an assisting-force applying portion operable to apply an assisting force to the brake for increasing the operating force of the brake to a value which is larger than a value corresponding to the output of the operating device by an amount of the assisting force.
The assisting force to be added to the brake by the assisting-force applying portion may be determined as desired. Where the brake-operating-force increasing device includes the abrupt braking control device described above with respect to the above modes (2) and (3), for instance, the assisting force may be determined to increase the operating force of the brake to a value larger than a value corresponding to an operating force which acts on a brake operating member and which is transmitted to the operating device as the input of the operating device. Where the brake-operating-force increasing device includes the braking-delay reducing device described above with respect to the above mode (4), the assisting force may be determined to be substantially equal to a difference between a value corresponding to the operating force of the brake operating member and a value corresponding to the actual value of the output of the operating device.
(10) A braking system according to the above mode (9), wherein the assisting-force applying portion is operable to control the assisting force to be applied to the brake such that the assisting force is larger when a deviation of an actual relationship between actual values of the input of the operating device from an ideal relationship between the input and the output is relatively larger than when the deviation is relatively small.
In the braking system according to the above mode (10), the assisting force to be added to the brake is relatively large when the operational delay of the operating device is relatively large. For instance, the assisting force is changed in steps corresponding to respective different ranges of the amount of the deviation of the actual input-output relationship of the operating device from the ideal relationship. Alternatively, the assisting force is continuously changed with a change of the amount of the deviation. In the latter case, the assisting force may be determined by multiplying the amount of the deviation by a predetermined coefficient, which may be a fixed constant or a variable that is changed continuously or in steps with an increase in the amount of the deviation.
Where the brake-operating-force increasing device includes the braking-delay reducing device, the assisting-force applying device having the feature according to the above mode (10) is obviously essential to minimize the adverse influence of the operational delay of the operating device. Where the brake-operating-force increasing device includes the abrupt braking control device, the assisting-force applying device according to the above mode (10) is operated to apply a larger assisting force to the brake when the operational delay of the operating device is relatively large, than when the operational delay is relatively small, since the relatively large operational delay is considered to indicate that the vehicle operator desires. a more abrupt brake application to the vehicle.
(11) A braking system according to the above mode (9) or (10), wherein the operating device is a vacuum booster operable to boost an input thereof into an output thereof by utilizing a negative pressure, and the assisting-force applying portion is operable to control the assisting force to be applied to the brake such that the assisting force is larger when a difference of a negative pressure from an atmospheric pressure is relatively small than when the difference of the negative pressure from the atmospheric pressure is relatively large.
When the negative pressure in the vacuum booster is relatively close to the atmospheric pressure, the amount of operational delay of the vacuum booster is relatively large than when the negative pressure is relatively close to the absolute vacuum. Accordingly, the operating force of the brake corresponding to a give operating force of the brake operating member is relatively small when the difference of the negative pressure from the atmospheric pressure is relatively small. Where the brake-operating-force increasing device includes the abrupt braking control device, the adverse influence of the operational delay of the booster can be eliminated or reduced by increasing the assisting force to be applied to the brake, with a decrease in the difference of the negative pressure with respect to the atmospheric pressure. This also applies to the braking system wherein the brake-operating-force increasing device includes the braking delay reducing device.
(12) A braking system according to any one of the above modes (9)-(11), wherein the assisting-force applying portion is operable to apply the assisting force to the brake when an amount of decrease of the input of the operating device within a predetermined time after a moment when the input has been increased to a maximal value is not smaller than a predetermined threshold.
The maximal value of the input of the operating device has been reached when the input which has been increasing begins to decrease. This change of the input from an increase to a decrease may be caused by a vehicle operator""s intentional operation of the brake operating member toward the non-operated position, or may take place contrary to the operator""s intention. As described below in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, the operating device may be a vacuum booster arranged to boost an input in the form of an operating force of a brake operating member manually operated by the vehicle operator, and transmit the boosted input to a master cylinder. A change of the input of the vacuum booster from an increase to a decrease may take place, for instance, when the operating force acting on the brake operating member is reduced after the brake operating member is once depressed by the operator with a relatively large force, because the operator cannot maintain the relatively large operating force, or for any other reason, contrary to the operator""s intention. In the braking system according to the above mode (12) wherein the assisting force is applied to the brake when the amount of decrease of the input of the booster is not smaller than the predetermined threshold, the amount of decrease of the operating force of the brake due to the decrease of the input of the booster can be reduced by the assisting force applied, as long as the amount of decrease of the input is not smaller than the threshold, so that the brake can be operated with a sufficiently large force, to provide a sufficiently large braking force for braking the vehicle. Where the decrease of the input of the booster larger than the threshold is caused by the operator""s intended releasing action of the brake operating member, the brake operating force is larger than desired. However, this case happens only infrequently, because the amount of decrease of the input of the booster is less likely to exceed the threshold within a predetermined time after the input has begun to decrease once it has increased to the maximal value. Even if the above case happened, an excessively large operating force of the brake would not have a detrimentally adverse effect on the running condition of the vehicle.
(13) A braking system according to the above mode (12), wherein said assisting-force applying portion is operable to control the assisting force to be applied to the brake such that the assisting force is larger when the amount of decrease of the input of the operating device within the predetermined time is relatively large than when the amount of decrease is relatively small.
(14) A braking system according to any one of the above modes (5)-(13), wherein the output-delay detecting portion is operable to detect the delay of the operating device when the input of the operating device has been increased to a predetermined value.
In the braking system according to the above mode (14), a requirement for an abrupt brake application to the vehicle is detected on the basis of the operational delay of the operating device, such that the requirement can be detected at a comparatively early point of time when the brake operating member is operated at a comparatively high speed by the vehicle operator to apply a brake in a comparatively abrupt manner. The requirement for an abrupt brake application to the vehicle can be detected by detecting the operational delay of the operating device when the actual output of the operating device has been increased to a predetermined value, for instance, on the basis of a difference between the actual value of the input detected when the actual output has been increased to the predetermined value, and the corresponding ideal value of the output which would be established if the operational delay were not present. In this case, however, the requirement for an abrupt brake application cannot be detected until the actual output of the operating device which is delay with respect to the actual input has been increased to the predetermined value. On the other hand, the actual input of the operating device does not include the operational delay, and increases at a rate corresponding to the amount of the operational delay. Accordingly, the time required for the actual input to be increased to the predetermined time decreases with an increase in the operating speed of the brake operating member, namely, with an increase in the degree of requirement for the abrupt brake application. Therefore, the output-delay detecting portion according to the above mode (14) permits the detection of the requirement for the abrupt brake application at a comparatively early point of time.
(15) A braking system according to the above mode (14), wherein the predetermined value of said input includes at least a first value and a second value larger than the first value.
The manner in which the operational delay of the operating device varies with the manner in which the operating device is operated (through the brake operating member, for instance). In the operating device is operated such that its input is increased at a relatively high rate, for example, the amount of operational delay of the operating device detected when the input has been increased to the predetermined first value is relatively large. If the operating device is operated such that its input is increased at a relatively low rate, the amount of operational delay detected when the input has been increased to the predetermined first value is relatively small. If the operating device is operated at a relatively high rate after the input has been increased to the predetermined first value, the amount of operational delay detected when the input has been increased to the predetermined second value is relatively large. The amount of operational delay of the operating device may be detected at three or more predetermined values of the input including the first and second values indicated above. The arrangement according to the above mode (15) wherein the amount of operational delay is detected at least two values of the input permits an increase of the operating force of the brake, if the amount of operational delay detected when the input has been increased to the predetermined second value or another value larger than the second value is larger than the predetermined upper limit, even if the amount of operational delay detected at the predetermined first value of the input is not larger than the upper limit.
(16) A braking system according to the above mode (9), wherein the assisting-force applying portion includes an ideal assisting-force applying portion (S120, S123) operable to apply the assisting force to the brake for thereby increasing the operating force of the brake by an amount of the assisting force, so as to establish an ideal relationship between the input and the operating force of the brake, which ideal relationship corresponds to an ideal relationship between the input and the output of the operating device.
In the braking system according to the above mode (16), the operating force of the brake is controlled to a value which is to be established when the input and output of the operating device has an ideal relationship therebetween. Namely, the assisting force is applied to the brake so that the operating force of the brake is increased by an amount of the assisting force, which is equal to the amount of shortage of the brake operating force due to the operational delay of the operating device.
(17) A braking system according to any one of the above modes (9)-(16), wherein the operating device is a vacuum booster operable to boost an input thereof into an output thereof by utilizing a negative pressure, and the assisting-force applying portion includes a post-boosting-limit assisting-force applying portion operable after a boosting limit of the vacuum booster has been reached, to apply the assisting force to the brake so that the operating force of the brake has a same relationship with the input of the operating device, as that before the boosting limit has been reached.
The vacuum booster is not capable of performing its boosting function after the pressure in the variable-pressure chamber has become equal to the atmospheric level, that is, after the boosting limit of the vacuum booster has been reached. After the boosting limit has been reached, the output of the booster is reduced with respect to a value which would be established if the input were continuously boosted as if the booster did not have the boosting limit. Accordingly, the output of the booster after the boosting limit has been reached is smaller than the ideal value as if the booster had an operational delay. In the braking system according to the above mode (17), the assisting force is applied to the brake after the boosting limit has been reached so that the operating force of the brake has a same relationship with the input of the booster, as that before the boosting limit has been reached. This arrangement eliminates the shortage of the brake operating force after the boosting limit of the vacuum booster has been reached.
(18) A braking system according to any one of the above modes (1)-(17), wherein the operating device is a vacuum booster operable to boost an input thereof into an output thereof by utilizing a negative pressure, the braking system further comprising a master cylinder operable by the output of the vacuum booster to deliver a pressurized fluid to the brake for operating the brake with the operating force corresponding to a pressure of the pressurized fluid.
(19) A braking system according to the above mode (18), wherein the brake-operating-force increasing device includes an assisting-force applying device operable to apply an assisting force to the brake for increasing the operating force of the brake to a value which is larger than a value corresponding to the output of the vacuum booster by an amount corresponding to the assisting force.
(20) A braking system according to the above mode (19), wherein the brake includes a wheel brake cylinder operable with a pressurized fluid for braking the wheel, and the assisting-force applying device includes a brake-cylinder-pressure control device operable to apply an assisting pressure to the wheel brake cylinder for increasing a pressure of the pressurized fluid delivered to the wheel brake cylinder, by an amount of the assisting pressure, so that the operating force of the brake is increased by the amount of the assisting force.
(21) A braking system according to the above mode (20), wherein the master cylinder includes a pressurizing piston, and the brake-cylinder-pressure control device includes an assisting-pressure applying portion which is disposed between the vacuum booster and the master cylinder and which is operable to apply the assisting pressure to the pressurizing piston so that the pressurizing piston receives the assisting force based on the assisting pressure, in addition to the output of said vacuum booster.
By application of the assisting pressure to the pressurizing piston of the master cylinder, a force corresponding to this assisting pressure is added to the pressurizing piston, in addition to the output of the vacuum booster, so that the pressure of the wheel brake cylinder is accordingly increased, and the operating force of the brake is accordingly increased.
(22) A braking system according to the above mode (21), wherein the assisting-force applying device (340, 370) includes:
an assisting cylinder portion (160) including an assisting cylinder (230, 238), and an assisting piston (238) fluid-tightly and sidably received in the assisting cylinder and cooperating with the assisting cylinder to define an assisting chamber (258) which receives the assisting pressure so that the pressure of the pressurized fluid to be delivered from the master cylinder to the wheel brake cylinder is increased by a amount corresponding to the assisting pressure; and
an assisting pressure control device (332) including a hydraulic pressure source (334, 336), and a pressure control valve (338) for controlling a pressure of a pressurized fluid delivered from the hydraulic pressure source and applying the controlled pressure to the assisting chamber as the assisting pressure so that the pressure of the pressurized fluid of the wheel brake cylinder is increased by the amount corresponding to the assisting pressure.
The assisting cylinder and the assisting piston of the assisting-cylinder portion may be formed integrally with the housing and the pressurizing piston of the master cylinder.
(23) A braking system according to the above mode (20), wherein the brake-cylinder-pressure control device is disposed between the master cylinder and the wheel brake cylinder, and is operable to increase the pressure of the wheel brake cylinder to a value higher than the pressure of the pressurized fluid delivered from the master cylinder.
In the braking system according to the above mode (23) wherein the assisting pressure is not applied to the assisting chamber, the assisting pressure is added to the pressure of the wheel brake cylinder to increase the operating force of the brake.
(24) A braking system according to claim 23, wherein the brake-cylinder-pressure control device includes:
a pump operable to deliver a pressurized fluid and connected to a fluid passage connecting the master cylinder and the wheel brake cylinder; and
a pressure control valve disposed in a portion of the fluid passage between the master cylinder and a point of connection of the fluid passage to the pump, the pressure control valve being operable to control the pressure of the pressurized fluid in the other portion of the fluid passage, so that the pressure of the wheel brake cylinder is higher than the pressure of the pressurized fluid as delivered from the master cylinder by an amount corresponding to the assisting force.
Where the delivery pressure of the pump is variable, the pressure control valve may be a simple solenoid-operated shut-off valve. In this case, the pressure of the pressurized fluid to be delivered to the wheel brake cylinder can be controlled by controlling the delivery pressure of the pump. The shut-off valve is kept in the closed state even after the fluid pressure of the wheel brake cylinder is made higher than the pressure of the master cylinder by the amount of the assisting pressure. Where the delivery pressure of the pump is not variable, the pressure control valve is preferably arranged to control the pressure of the wheel brake cylinder by controlling the amount of electric current to be applied thereto. In this case, the amount of electric current to be applied to the pressure control valve is controlled such that a portion of the pressurized fluid delivered from the pump is returned to the master cylinder through the pressure control valve so that the pressure of the wheel brake cylinder is made higher than the pressure of the master cylinder by the amount of the assisting pressure. Where the delivery pressure of the pump is not variable and the pressure control valve is a simple solenoid-operated shut-off valve, a second pressure control valve may be provided in a portion of the fluid passage between the above-indicated simple solenoid shut-off valve and the wheel brake cylinder or the pump, so that the pressure of the wheel brake cylinder is controlled by controlling the amount of electric current to be applied to the second pressure control valve. In this case a portion of the pressurized fluid delivered from the pump is returned to a reservoir, rather than to the master cylinder. This reservoir may be a master reservoir from which the fluid is supplied to the master cylinder, or a pressure-reducing reservoir to which the pressurized fluid is discharged from the wheel brake cylinder when the pressure of the wheel brake cylinder is reduced. In either one of the above cases, the pump is preferably arranged to pressurize the fluid received from the master reservoir or pressure-reducing reservoir to which the fluid is discharged through the pressure control valve to control the pressure of the wheel brake cylinder. However, this arrangement is not essential.